US 7441324 B2
A method is provided for preserving the transverse biasing of a GMR (or MR) read head during back-end processing. In a first preferred embodiment, the method comprises magnetizing the longitudinal biasing layers of the read head in a transverse direction, so that the resulting field at the position of the transverse biasing layer places it in a minimum of potential energy which stabilizes its direction. The field of the longitudinal biasing layer is then reset to the longitudinal direction in a manner which maintains the transverse biasing direction. In a second embodiment, a novel fixture for mounting the read head during processing includes a magnetic portion which stabilizes the transverse bias of the read head. The two methods may be used singly or in combination.
1. A method for preserving a transverse biasing of a read head during back-end processing of a workpiece which includes said read head, comprising:
providing said workpiece to be processed, said workpiece including at least one transversely biased read head, and said read head also including longitudinal biasing layers;
magnetizing said longitudinal biasing layers in a transversing direction whereby the magnetizing provides a transverse magnetic field to said longitudinal biasing layers that is oriented in a direction parallel to a transverse biasing field of said read head;
back-end processing said workpiece;
mounting said workpiece on a bottom surface of a fixture, said fixture thereby firmly holding said workpiece during said back-end processing, said fixture comprising a first portion formed of material which is not permanently magnetic, and a second portion which is magnetized and whose magnetic field is oriented in a direction parallel to the transverse biasing field of said read head and thereby, in combination with the transverse magnetic field of said longitudinal biasing layer, said second portion of said fixture stabilizes said transverse biasing field of said read head in a state of minimum potential energy;
after said step of back-end processing, re-setting the magnetization of said longitudinal biasing layers in a longitudinal direction by placing said read head in an appropriate external longitudinal magnetic field.
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This is a division of patent application Ser. No. 10/394,084, filing date Mar. 21, 2003, now matured into U.S. Pat. No. 6,922,884, A Method For Preventing Magnetic Damage To A GMR Head During Back-End Processing, assigned to the same assignee as the present invention, which is herein incorporated by reference in its entirety.
1. Field of the Invention
This invention relates generally to “back-end” processing of GMR read heads, including the processing of assemblies containing GMR read heads, and particularly to a method of preventing damage to the transverse biasing of the GMR read heads by process induced stresses and by external magnetic fields that may be present during such processing.
2. Description of the Related Art
GMR (giant magneto-resistive) and MR (magneto-resistive) heads and slider assemblies are formed on wafers in arrays of complete, already magnetized units, which must then be subjected to additional, so-called “back-end” process steps, such as separation of the array into individual units and lapping each unit to an acceptable degree of smoothness. An example of how at least a substantial portion of such “back-end” processing proceeds is provided by Sasaki et at. (U.S. Pat. No. 6,374,479), who teach a method of slicing the wafer into rows of slider sections and of bonding the sliced sections to a supporting plate for further processing.
The heads, themselves, are small and delicate and subject to various types of damage during the back-end processing steps. The industry has been particularly concerned with damage to the heads caused by electrostatic discharges (ESD) that can occur during the processing. In this regard, Girard et al. (U.S. Pat. No. 6,146,813) teach a method of forming (and removing) shunts between portions of electrical components (including GMR heads), said shunts providing a mechanism for safely dissipating induced currents and electrostatically deposited charges. Han et al. (U.S. Pat. No. 6,415,500) teaches a method of avoiding ESD by connecting the sensor to its shields during the back-end processing steps in such a way that there is an equal electrical potential between the sensor and its shields during the duration of the process.
Another concern during back-end processing is that the lapping process can proceed beyond the desired limits and damage the active surfaces of the GMR head. Zhu (U.S. Pat. No. 6,230,389) teaches the formation of a lapping monitor, which is an additional, sacrificial portion of the sensor layer whose changing shape during lapping allows the progress of the lapping to be accurately followed.
None of the methods discussed above address the problem of possible adverse affects of back-end processing to the magnetic properties of the GMR head. Even before the beginning of back-end processing, the magnetic properties of GMR layers have been established by annealing in the presence of appropriate magnetic fields. Magnetic biasing is of particular importance to the performance of a GMR head and two types of biasing are established prior to back-end processing: longitudinal and transverse. Longitudinal biasing, typically provided by adjacent permanent magnetic layers formed with the conducting lead layers, stabilizes the domain structure and magnetic moment direction of a GMR head's free layer. Transverse biasing, typically provided by a antiferromagnetically pinned layer formed within the GMR sensor, provides a reference direction with respect to which the magnetic moment of the free layer moves. Both of these biasing structures are already established during the wafer formation prior to back-end processing.
In back-end processing, the lapping process produces stresses and even plastic deformations within the wafer as an unavoidable part of stock removal. These stresses cause both biases to change, particularly when combined with other disturbances such as stray magnetic fields, elevated temperatures and ESD induced currents. As a result, the final GMR product can have its biases altered in a random and uncontrollable fashion, adversely affecting product yields. While the longitudinal bias can be restored to its pre-processing state with relative ease, restoration of the transverse bias cannot be accomplished economically either during or after the processing. Prior art methodology, as noted above, has concentrated on prevention of ESD events during processing. The present invention teaches an entirely new method for controlling damage to biasing during back-end processing and, by so doing, will improve product yields substantially.
Accordingly, it is a first object of this invention to provide a method of improving GMR (and, generally, MR) product yields by restoring magnetic biasing during and subsequent to back-end processing to within specifications established prior to back-end processing.
It is a second object of the present invention to provide a novel fixture to support GMR assemblies during back-end processes such as row and slider manufacturing operations including lapping, which fixture enables the use of external magnetic fields to stabilize the transverse biasing in said assemblies.
The objects stated above will be achieved by applying the proper magnetic field to the GMR head (or other assembly, generically called the “workpiece” hereinafter) during back-end processing. The field can be provided by permanent or electro-magnets which can be external to the workpiece and mounted in a fixture as discussed below, or incorporated within the head structure itself. The magnitude and direction of the field is such that the desired (ie., pre-processing) state of transverse bias is a state of minimum potential energy. More specifically, the direction of the field should be parallel to the desired exchange field provided by the antiferromagnetic (AFM) pinning layer which pins the transverse biasing layer. This field will hereinafter be referred to as the “stabilizing field.” Because the AFM structure is situated in a state of minimum potential energy, it can resist disturbances such as mechanical stress, electrical shock, temperature elevation and stray magnetic fields. Consequently, the transverse bias of the GMR structure is less likely to change.
At least a part of the stabilizing field can be supplied by the permanent magnets which are used to provide longitudinal bias. Although these magnets are intended to provide the necessary longitudinal fields, they can be initially magnetized so as to stabilize the transverse bias and then restored to their intended longitudinal direction after back-end processing is complete. Specifically, this requires the permanent magnets to be initially oriented so that they oppose the exchange field of the AFM layer. In this orientation, the external stabilizing field is parallel with the exchange field. Subsequent to the back-end process, the longitudinal bias is set back to its proper value and direction using, for example, a process such as the following: 1) subject the workpiece to a longitudinal magnetic field of approximately 5000 Oe, for a predetermined time such as approximately 10 seconds; 2) then, subject the workpiece to a decreasing, alternating transverse magnetic field, varying in magnitude from 5000 Oe to 100 Oe. In this process, the imposed longitudinal field restores the longitudinal bias, but hysteresis effects will prevent the transverse biasing field from attaining its most stable state. The oscillating, alternating transverse field allows this stable final state to be attained. The limiting values of the oscillating field (ie., 500 Oe and 100 Oe) are chosen so that the higher value will not damage the re-set longitudinal bias and so that the lower value is comparable to the largest field that a GMR head would experience during normal operating conditions.
Although a substantial portion of, even the entire, stabilization field may be provided by the longitudinal biasing magnets, stabilization may also be advantageously provided by external magnets forming part of a novel fixture which simultaneously holds the workpiece and stabilizes its transverse bias. The design of such a fixture and the effects of its imposed magnetic fields will be further discussed and described below under the description of the preferred embodiments.
The preferred embodiments of the present invention each provide a method for preserving the desired transverse biasing of an MR or GMR read head during back-end processing of the read head. In a first preferred embodiment, the transverse biasing field is stabilized by applying an additional, stabilizing field to the read head which results in the biasing layer being in a region of minimum potential energy when the biasing field is properly oriented in a transverse direction. In this embodiment, the stabilizing field is internally provided, by the longitudinal biasing permanent magnetic layer of the read head. This necessitates the field of the longitudinal biasing being re-oriented in a transverse direction prior to processing and is being re-set to the required longitudinal direction when the processing steps are completed. The re-setting process is achieved by first placing the read head in a strong longitudinal field for a brief time period, following which it is placed in a decreasing, oscillating transverse field, to insure stability of the longitudinal biasing layers.
In a second preferred embodiment, the stabilizing field is externally supplied by affixing the read head (or whatever workpiece contains the read head) to a fixture that incorporates a magnetic portion. The fixture simultaneously holds the workpiece for the processing steps (eg. lapping), while providing the magnetic field necessary to maintain the proper transverse bias direction. The magnetic portion of the fixture may be configured in at least two ways: 1) directly between the body of the fixture (which may be of soft magnetic material or may be non-magnetic) and the workpiece, or, 2) alongside the workpiece, but not in contact with it. In either configuration (and others may be envisioned) the magnetic portion provides the necessary magnetic field.
It is to be recognized that the methods of each embodiment may be used by themselves or in combination. For example, if the desired stabilization of the biasing can be obtained using only the field of the longitudinal biasing layer, the workpiece may be mounted on a standard fixture of the prior art. If additional stabilization is required, the workpiece may be mounted on the fixture of the second preferred embodiment and, in addition, the longitudinal biasing layer may be transversely oriented to provide additional stabilization.
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The diagram also shows the novel magnetization direction of both of the longitudinal biasing layers, which is directed transversely to the cross-sectional plane, as shown by the outward directed arrow (dot in circle (80)). In the prior art, the magnetization would be directed longitudinally, in the plane of the cross-section and parallel to the free layer (40), at this stage of the processing. In the present method, the magnetization of the longitudinal biasing layers is set opposite to (antiparallel to) the magnetization of the pinned layer, which is shown as an arrow directed into the plane of the cross-section (90). There is also shown the magnetization direction of the AFM layer (95), which exchange couples to that of the pinned layer.
At this time, the read head, which would typically be part of a more complex workpiece, will be subject to processing steps that could, in the methods of the prior art, adversely affect the magnetization (90) of the pinned layer. In the present method, however, the combination of the magnetization of the biasing layers (80) and of the pinning layer (95), places the pinned layer in a potential energy minimum when its magnetization is transversely directed as required. This energetically favorable condition makes it much more unlikely that the magnetization of the pinned layer will change during processing. It is noted that the transverse fields of the longitudinal biasing layers may be inadequate to provide the necessary stabilization of the transverse biasing under certain processing steps or given size restrictions on the biasing layers themselves. In this case, additional, external magnets may be required to augment the fields provided by the internal magnets which are the longitudinal biasing layers.
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As is understood by a person skilled in the art, the preferred embodiments of the present invention are illustrative of the present invention rather than limiting of the present invention. Revisions and modifications may be made to methods, materials, structures and dimensions employed in providing a method for preserving the transverse biasing of an MR or GMR read head during back-end processing of the read head, while still providing such a a method for preserving the transverse biasing of an MR or GMR read head during back-end processing of the read head as described herein, in accord with the spirit and scope of the present invention as defined by the appended claims.